1. Field of the Invention
The present invention relates to processing systems in general and, more specifically, to parallel processing architectures.
2. Description of the Related Art
Many computing tasks can be developed that operate in parallel on data. The efficiency of the parallel processor depends upon the parallel processor's architecture, the coded algorithms, and the placement of data in the parallel elements. For example, image processing, pattern recognition, and computer graphics are all applications which operate on data that is naturally arranged in two- or three-dimensional grids. The data may represent a wide variety of signals, such as audio, video, SONAR or RADAR signals, by way of example. Because operations such as discrete cosine transforms (DCT), inverse discrete cosine transforms (IDCT), convolutions, and the like which are commonly performed on such data nay be performed upon different grid segments simultaneously, multiprocessor array systems have been developed which, by allowing more than one processor to work on the task at one time, may significantly accelerate such operations. Parallel processing is the subject of a large number patents including U.S. Pat. Nos. 5,065,339; 5,146,543; 5,146,420; 5,148,515; 5,546,336; 5,542,026; 5,612,908 and 5,577,262; European Published Application Nos. 0,726,529 and 0,726,532 which are hereby incorporated by reference. One conventional approach to parallel processing architectures is the nearest neighbor mesh connected computer, which is discussed in R. Cypher and J. L. C. Sanz, SIMD Architectures and Algorithms for Image Processing and Computer Vision, IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. 37, No. 12, pp. 2158-2174, December 1989; K. E. Batcher, Design of a Massively Parallel Processor, IEEE Transactions on Computers, Vol. C-29 No. 9, pp. 836-840 September 1980; and L. Uhr, Multi-Computer Architectures for Artificial Intelligence, New York, N.Y., John Wiley & Sons, Ch. 8, p. 97, 1987.
In the nearest neighbor torus connected computer of FIG. 1A multiple processing elements (PEs) are connected to their north, south, east and west neighbor PEs through torus connection paths MP and all PEs are operated in a synchronous single instruction multiple data (SIMD) fashion. Since a torus connected computer may be obtained by adding wraparound connections to a mesh-connected computer, a mesh-connected computer, one without wraparound connections, may be thought of as a subset of torus connected computers. As illustrated in FIG. 1B, each path MP may include T transmit wires and R receive wires or as illustrated in FIG. 1C, each path MP may include B bidirectional wires. Although unidirectional and bidirectional communications are both contemplated by the invention, the total number of bus wires, excluding control signals, in a path will generally be referred to as k wires hereinafter, where k=B in a bidirectional bus design and k=T+R in a unidirectional bus design. It is assumed that a PE can transmit data to any of its neighboring PEs, but only one at a time. For example, each PE can transmit data to its east neighbor in one communication cycle. It is also assumed that a broadcast mechanism is present such that data and instructions can be dispatched from a controller simultaneously to all PEs in one broadcast dispatch period.
Although bit-serial inter-PE communications are typically employed to minimize wiring complexity, the wiring complexity of a torus-connected array nevertheless presents implementation problems. The conventional torus-connected array of FIG. 1A includes sixteen processing elements connected in a four by four array 10 of PEs. Each processing element PEi,j is labeled with its row and column number i and j, respectively. Each PE communicates to its nearest North (N), South (S), East (E) and West (W) neighbor with point to point connections. For example, the connection between PE0,0 and PE3,0 shown in FIG. 1A is a wraparound connection between PE0,0's N interface and PE3,0's south interface, representing one of the wraparound interfaces that forms the array into a torus configuration. In such a configuration, each row contains a set of N interconnections and, with N rows, there are N2 horizontal connections. Similarly, with N columns having N vertical interconnections each, there are N2 vertical interconnections. For the example of FIG. 1A, N=4. The total number of wires, such as the metallization lines in an integrated circuit implementation in an N×N torus-connected computer including wraparound connections, is therefore 2 kN2, where k is the number of wires in each interconnection. The number k may be equal to one in a bit serial interconnection. For example with k=1 for the 4×4 array 10 as shown in FIG. 1A, 2 kN2=32.
For a number of applications where N is relatively small, it is preferable that the entire PE array is incorporated in a single integrated circuit. The invention does not preclude implementations where each PE can be a separate microprocessor chip, for example. Since the total number of wires in a torus connected computer can be significant, the interconnections may consume a great deal of valuable integrated circuit “real estate”, or the area of the chip taken up. Additionally, the PE interconnection paths quite frequently cross over one another complicating the IC layout process and possibly introducing noise to the communications lines through crosstalk. Furthermore, the length of wraparound links, which connect PEs at the North and South and at the East and West extremes of the array, increase with increasing array size. This increased length increases each communication line's capacitance, thereby reducing the line's maximum bit rate and introducing additional noise to the line.
Another disadvantage of the torus array arises in the context of transpose operations. Since a processing element and its transpose are separated by one or more intervening processing elements in the communications path, latency is introduced in operations which employ transposes. For example, should the PE2,1 require data from its transpose, PE1,2, the data must travel through the intervening PE1,1 or PE2,2. Naturally, this introduces a delay into the operation, even if PE1,1 and PE2,2 are not otherwise occupied. However, in the general case where the PEs are implemented as micro-processor elements, there is a very good probability that PE1,1 and PE2,2 will be performing other operations and, in order to transfer data or commands from PE1,2 to PE2,1, they will have to set aside these operations in an orderly fashion. Therefore, it may take several operations to even begin transferring the data or commands from PE1,2 to PE1,1 and the operations PE1,1 was forced to set aside to transfer the transpose data will also be delayed. Such delays snowball with every intervening PE and significant latency is introduced for the most distant of the transpose pairs. For example the PE3,1/PE1,3 transpose pair of FIG. 1A, has a minimum of three intervening PEs, requiring a latency of four communication steps and could additionally incur the latency of all the tasks which must be set aside in all those PEs in order to transfer data between PE3,1 and PE1,3 in the general case.
Recognizing such limitations of torus connected arrays, new approaches to arrays have been disclosed in U.S. Pat. No. 5,612,908; A Massively Parallel Diagonal Fold Array Processor, G. G. Pechanek et al., 1993 International Conference on Application Specific Array Processors, pp. 140-143, Oct. 25-27, 1993, Venice, Italy, and Multiple Fold Clustered Processor Torus Array, G. G. Pechanek, et. al., Proceedings Fifth NASA Symposium on VLSI Design, pp. 8.4.1-11, Nov. 4-5, 1993, University of New Mexico, Albuquerque, N. Mex. which are incorporated by reference herein in their entirety. The operative technique of these torus array organizations is the folding of arrays of PEs using the diagonal PEs of the conventional nearest neighbor torus as the foldover edge. As illustrated in the array 20 of FIG. 2, these techniques may be employed to substantially reduce inter-PE wiring, to reduce the number and length of wraparound connections, and to position PEs in close proximity to their transpose PEs. This processor array architecture is disclosed, by way of example, in U.S. Pat. Nos. 5,577,262, 5,612,908, and EP 0,726,532 and EP 0,726,529 which were invented by the same inventor as the present invention and are incorporated herein by reference in their entirety. While such arrays provide substantial benefits over the conventional torus architecture, due to the irregularity of PE combinations, for example in a single fold diagonal fold mesh, some PEs are clustered “in twos”, others are single, in a three fold diagonal fold mesh there are clusters of four PEs and eight PEs. Due to an overall triangular shape of the arrays, the diagonal fold type of array presents substantial obstacles to efficient, inexpensive integrated circuit implementation. Additionally, in a diagonal fold mesh as in EP 0,726,532 and EP 0,726,529, and other conventional mesh architectures, the interconnection topology is inherently part of the PE definition. This fixes the PE's position in the topology, consequently limiting the topology of the PEs and their connectivity to the fixed configuration that is implemented. Thus, a need exists for further improvements in processor array architecture and processor interconnection.